1. Field of the Invention
The present invention relates to a semiconductor manufacturing process, and more specifically to a process of fabricating a high resistance CMOS resistor.
2. Description of Related Art
Many common circuits manufactured using CMOS process require resistor elements. In the area of analog circuits, resistor elements having high-resistance values (100 kΩ–500 kΩ) are sometimes desired. The demand for such resistors is especially common in the field of power-relative analog circuits.
In one common application, many power-relative analog circuits include voltage dividers constructed from a pair of CMOS resistors. Such voltage dividers provide reference voltages that are stepped down from a supply voltage. The higher the resistance of the resistors is, the less standby power will be consumed.
Currently, most widely used CMOS resistors are either junction resistors or film resistors. Common junction resistor types include NW resistors, PW resistors, NDIFF(N+S/D) resistors, and PDIFF(N+S/D) resistors. Common film resistor types include Poly resistors, M1 resistors, and M2 resistors.
Though these resistors have satisfactory resistor characteristics, their resistance is generally limited to 1 kΩ–5 kΩ per square. When a high-resistance element is required, it can be created either by using extra die space, or by performing additional masking steps during the manufacturing process. However, neither of these alternatives is desirable because they both increase the cost of manufacturing the circuit.
Creating a high-resistance resistor (100 kΩ–500 kΩ) with traditional CMOS resistors is often commercially impractical due to die-size and cost restrictions. Therefore, there exists a need for a CMOS resistor having a significantly higher resistance per square than existing CMOS resistors. Furthermore, the process for manufacturing such a CMOS resistor should not require any additional masking steps.